The market for high strength post insulators is dominated by porcelain because they can offer a large diameter solid core insulator at a low cost. Solid core composite insulators with large diameters are difficult to make and more expensive than porcelain. The composite insulator industry has struggled to achieve cost efficient solutions using hollow core insulators where the inside insulation is based on foam filling or high pressure gas filling.
The invention relates to such post insulators of any size used for separating two electrical potentials, normally a high electrical potential from ground. They may be used as so called station post insulators in switchgears in converter stations of plants for transmitting electric power, such as for separating valves in a converter in a station of a HVDC (High Voltage Direct Current) plant with respect to ground. Another possible use is to carry overhead electrical high voltage cables.
Post insulators can have specific lengths, i.e. heights, of 2-12 m, but any other size is possible.
The voltage, i.e. potential difference, in question may for instance be 100-1200 kV, although quite different voltages are possible. The voltage may be an alternating voltage or a direct voltage.
It is very important in a post insulator of this type to prevent short circuits between the electrical potentials separated by the insulator. The outside of the insulator is normally covered in sheds and the insulator is designed to withstand the voltage difference on the outside of the insulator. In a gas-filled composite insulator, the inside volume is normally a controlled atmosphere at the time of first use but there is a risk that over time moisture will leak into the enclosed volume of the insulator, due to leaks in the end sealing which could lead to short circuits. In high pressure gas filled applications the pressure is usually monitored and such leaks will be detected by a pressure drop.
EP 1 801 819 discloses a foam filled post insulator. The problem with foam filled post insulators is that they need to have a strong and durable interface between the tube and foam core to avoid water forming an axial conductor in the cavities of a weak interface where the foam separated from the insulator tube. CN 2011/56452U discloses another post insulator.
JP 09139128 discloses a gas filled post insulator with airtight end sealing and electric field relaxing parts, projecting into the tube but the electric field relaxing part is arranged with holes and the insulator contains moisture absorbent parts.
GB 635,814 relates to gas-filled electric insulators and has for one of its objects to provide means whereby the insulator may be replenished with gas. A hollow electric gas filled insulator has permanently sealed in one end thereof a bush which is internally threaded for receiving a removable threaded plug. Preferably the plug is provided with a head overlying the outer face of the bush and a sealing washer is disposed between the head and the bush. The aforesaid bush and plug may be arranged to be inset from the end of the insulator so that a metal fitting to be secured to said end may extend clear of the plug. For example, the aforesaid metal fitting may comprise a cap provided with a removable cover plate so that ready access may be obtained for the removal of said plug.